1. Field of the Invention
The present invention relates to an image forming apparatus capable of forming an image on a large-size transfer sheet.
2. Description of the Prior Art
Electrophotographic copying machines are widely used which are adapted to scan a document original under light irradiation, form an electrostatic latent image on a photoreceptor by light rays reflected on the document original, develop the electrostatic latent image into a toner image, and thermally fix the toner image on a transfer sheet. Some of these copying machines are capable of copying a large-size document original such as of JIS A0 size.
The copying machines for copying a large-size document original have a reading mechanism capable of reading a large-size document original and a transporting mechanism capable of transporting a transfer sheet of a large size corresponding to the size of the document original.
When such a large-size transfer sheet is transported, the transfer sheet is liable to be biased, resulting in transportation failure (so-called jam) of the transfer sheet.
Further, when the transfer sheet is transported from one transportation roller to the next transportation roller, there is a tendency to form a distorted copy image.
The transfer sheet traveling speed relative to the circumferential speed of the photoreceptor should be constant when a toner image on the photoreceptor is transferred onto the transfer sheet. If the transfer sheet traveling speed relative to the circumferential speed of the photoreceptor is changed, the scale of an image to be copied is changed in a transfer sheet transportation direction. Where a large-size transfer sheet is transported, it is difficult to keep the transfer sheet traveling speed relative to the circumferential speed of the photoreceptor constant because of the structure of the copying machine. This is because a transportation speed at which the transfer sheet is taken into a fixing unit from the photoreceptor for fixing the toner image on the transfer sheet is generally set higher than a transportation speed at which the transfer sheet is fed into the photoreceptor. Where a larger-size transfer sheet is used, the rearward portion of the transfer sheet does not reach the photoreceptor, when the leading edge of the transfer sheet enters the fixing unit. Therefore, the scale of an image to be copied on the transfer sheet is changed during the transportation of the transfer sheet.
For the foregoing reason, there is a need to prevent the scale of an image to be copied on the transfer sheet from being changed.
Additionally, there is a similar problem to be solved in image forming apparatuses other than copying machines, for example, printing machines for printing an image on a larger-size sheet.
In view of the foregoing problem, it is one object of the present invention to provide an image forming apparatus comprising a transportation mechanism capable of properly transporting a large-size transfer sheet.
It is another object of the present invention to provide an image forming apparatus which is so improved as to prevent a transfer sheet from being biasedly transported.
It is still another object of the present invention to provide an image forming apparatus which is so improved as not to distort an image to be transferred onto a transfer sheet nor change the scale of the image even if the transfer sheet transportation speed relative to the circumferential speed of the photoreceptor is changed during the transportation of the transfer sheet.